Synthesis gas (hereinafter referred to as syngas) is a mixture of hydrogen (H2) and carbon monoxide (CO). Syngas can be produced, in principle, from virtually any material containing carbon. Carbonaceous materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite; and renewable resources such as lignocellulosic biomass and various carbon-rich waste materials.
There exist a variety of conversion technologies to turn these feedstocks into syngas. Conversion approaches can utilize a combination of one or more steps comprising gasification, pyrolysis, steam reforming, and/or partial oxidation of a carbon-containing feedstock.
Syngas is a platform intermediate in the chemical and biorefining industries and has a vast number of uses. Syngas can be converted into alkanes, olefins, oxygenates, and alcohols. These chemicals can be blended into, or used directly as, diesel fuel, gasoline, and other liquid fuels. Syngas can also be directly combusted to produce heat and power.
Today, almost half of all gasoline sold in the United States contains ethanol (American Coalition for Ethanol, www.ethanol.org, 2006). The ethanol in gasoline and other liquid fuels raises both the oxygen and the octane content of the fuels, allowing them to burn more efficiently and produce fewer toxic emissions.
It is preferable to utilize a renewable resource to produce ethanol because of the rising economic, environmental, and social costs associated with fossil resources. Calculations show that when renewable feedstocks, such as biomass, are converted into syngas using techniques described above, a selective process for converting this syngas into ethanol has the theoretical potential to produce approximately 200 gallons of ethanol per ton of biomass. No publicly known process, however, can achieve such yields of ethanol.
In light of the state of the art, what is needed is a method, as well as an apparatus to carry out the method, to improve the selectivity and yield to ethanol. Specifically, there exists a need to overcome the carbon loss to methanol, propanol, and higher alcohols, when ethanol is desired. Additionally, a need exists to reduce the carbon loss to the water-gas shift reaction that consumes CO and generates CO2.